The present invention relates to a drilling system for drilling a borehole into an earth formation.
In the typical drilling system, an annular space is formed between the drill string extending into the borehole and the borehole wall. This annular space usually contains a body of drilling fluid. One of the purposes of the fluid in the annular space is to control the pressure at the wellbore wall, which pressure normally is kept between an allowable upper limit depending on the fracturing pressure of the rock formation and an allowable lower limit depending on the pore pressure of the formation fluid. The fluid pressure in the annular space is determined by the hydrostatic weight of the fluid column in the annular space, and by a dynamic pressure component which depends on the flow resistance of the drilling fluid in the annular space as the drilling fluid flows from the borehole bottom back to surface. The pressure is normally controlled by applying selected weighting material in the drilling fluid.
In the prior art it has been practiced to drill wellbores at wellbore pressures close to the lower limit, with the advantage that the risk of damage to the rock formation is reduced. Such applications are referred to as at-or under-balanced drilling whereby lighter drilling fluids than normal are applied. During tripping of the drill string out of the borehole or lowering the drill string into the borehole, the individual drill string sections are disconnected from each other so that no longer fluid can be pumped via the drill string into the annular space. During such tripping or lowering of the drill string, a problem arises in that the dynamic pressure component vanishes since no longer drilling fluid flows from the borehole bottom to surface. As a result the fluid pressure in the annular space can become lower than the allowable lower limit, potentially leading to undesired fluid influx from the earth formation into the borehole.
It is an object of the invention to alleviate the problem of the prior art and to provide a drilling system which can be safely used without the danger of undesired fluid influx from the earth formation into the borehole, even for at-and under-balanced drilling.
In accordance with the invention there is provided a drilling system for drilling a borehole into an earth formation, the drilling system comprising
a drill string extending into the borehole whereby an annular space is formed between the drill string and the borehole wall, the annular space containing a body of fluid, the drill string including a longitudinal fluid passage having an outlet opening at the lower end part of the drill string;
pumping means for selectively pumping drilling fluid via said passage and outlet opening into the body of fluid; and
a fluid discharge conduit for discharging fluid from the body of fluid; wherein the drilling system further comprises pressure control means for controlling the fluid pressure in the body of fluid when the pumping means is inoperative to pump drilling fluid into the body of fluid.